Fig.42.9 ).AbnormallyhighRAIU(>60%)after24hoursindicateshyperthyroidism.AbnormallylowRAIU(<5%)after24hoursindicateshypothyroidism.Inindividualswithextremechronicstimulationofthethyroid(Gravesdisease–associatedthyrotoxicosis),iodideistrapped,organified,andreleasedashormoneveryrapidly.Inthesecasesofelevatedturnover,6-hourRAIUwillbeveryhighbut24-hourRAIUwillbelower(see
Fig.42.8 ).Anumberofanions,suchasthiocyanate(CNS−),perchlorate(HClO4−),andpertechnetate(TcO4−),arecompetitiveornoncompetitiveinhibitorsofiodidetransportviaNIS.Ifiodidecannotberapidlyincorporatedintotyrosine(organification defect) afteritsuptakebythecell,administrationofoneoftheseanionswill,byblockingfurtheriodideuptake,causerapidreleaseofiodidefromthegland(see
Fig.42.9
Thisreleaseoccursasaresultofthehighthyroid-plasmaconcentrationgradient.
Thethyroidcanbeimagedwitharectilinearscannerorgammacameraafteradministrationofatracer,123I,131I,ortheiodine-mimicpertechnetate(99mTc).Imagingcandisplaythesizeandshapeofthethyroid(see
Fig.42.1C )aswellasheterogeneitiesofactiveversusinactivetissuewithinthethyroidgland.Suchheterogeneitiesareoftenduetothedevelopmentofthyroid nodules, whichareregionsofenlargedfollicleswithevidenceofregressivechangesduetocyclesofstimulationandinvolution.“Hot” nodules (i.e.,nodulesthatdisplayhighRAIUonimaging)arenotusuallycancerousbutmayleadtothyrotoxicosis(hyperthyroidism).“Cold” nodules are10timesmorelikelytobecancerous.Suchnodulescanbesampledforpathologicalanalysisbyfine-needle aspiration biopsy.
Thethyroidcanalsobeimagedbyultrasonography, whichissuperiorinresolutiontoRAIUimaging.Ultrasonographyisusedtoguidethephysicianduringfine-needleaspirationbiopsyofanodule.Thehighestresolutionofthethyroidisachievedwithmagnetic resonance imaging (MRI).
A B Colloid in endocytic vesicles Colloid in lumen of thyroid follicle •Fig. 42.7 Before(A)andminutesafter(B)rapidinductionofthy-roglobulinendocytosisbyTSH.(FromWollmanSHetal.J Cell Biol 1964;21:191.)
TBG concentration significantly affect total plasma T4 and T3 levels. Two important biological functions have been ascribed to TBG. First, it maintains a large circulating reservoir of T4 that buffers any acute changes in thyroid gland function. Second, binding of plasma T4 and T3 to proteins prevents loss of these relatively small hormone molecules in urine and thereby helps conserve iodide. TTR transports T4 in cerebrospinal fluid and provides thyroid hormones to the CNS.
Regulation of Thyroid Function